Cyanobacteria, often called blue-green algae, are ancient photosynthetic organisms found in nearly every environment on Earth, from oceans and freshwater to deserts and hot springs. These microbes have been a driving force in the planet’s history, using sunlight to produce their own food and influencing global ecosystems. Understanding their structure is key to classifying them, but their morphology exhibits a great deal of diversity.
The Answer: Single Cells, Colonies, or Filaments?
Cyanobacteria are fundamentally unicellular organisms because each cell is a complete, self-sufficient prokaryotic unit. They do not exhibit the extensive tissue specialization found in true multicellular organisms like plants or animals. Their classification becomes complex due to the varied ways these single cells aggregate in nature.
Many cyanobacteria exist as true unicellular forms, such as the globally abundant Prochlorococcus, which is a solitary cell floating freely in the water. Other species form colonies, where individual cells are encased within a shared, gelatinous sheath or mucilage.
Within these colonies, each cell performs all necessary life functions independently. This means the group acts as an aggregate rather than a single coordinated organism.
A third common arrangement is the filamentous form, where cells are lined up end-to-end, forming hair-like chains called trichomes. These filaments can display a simple form of cell differentiation, blurring the line between colonial and true multicellular life.
For instance, some filamentous species develop specialized, thick-walled cells called heterocysts, which are dedicated to fixing atmospheric nitrogen. The presence of these specialized cells, which depend on neighboring vegetative cells for carbon, represents a basic level of functional interdependence.
Distinctive Internal Biology of Cyanobacteria
Cyanobacteria are taxonomically classified as bacteria, meaning they are prokaryotes and lack a membrane-bound nucleus to house their genetic material. Unlike eukaryotic algae, cyanobacteria do not contain mitochondria, a Golgi apparatus, or chloroplasts for photosynthesis.
Instead, the chlorophyll and other pigments necessary for photosynthesis are embedded in internal membrane structures called thylakoids. These thylakoid membranes are flattened sacs that float freely within the cytoplasm, serving as the site for light-dependent reactions. This internal organization is a key distinction, as chloroplasts in eukaryotes are thought to have evolved from an ancient cyanobacterium through endosymbiosis.
Some filamentous cyanobacteria also form specialized cells for survival, such as akinetes. These are dormant, thick-walled cells that are resistant to harsh environmental conditions, allowing the organism to survive periods of drought or freezing.
Why Their Presence Matters: Ecological Roles and Blooms
The ecological significance of cyanobacteria is immense, stemming from their capacity to perform oxygenic photosynthesis. Approximately 2.4 billion years ago, these organisms were responsible for the Great Oxygenation Event, fundamentally changing the Earth’s early atmosphere by pumping massive amounts of oxygen into it. This event paved the way for the evolution of all oxygen-breathing life forms.
Today, they continue to play a positive role through nitrogen fixation, converting inert atmospheric nitrogen gas into forms that can be used by other organisms. This process acts as a natural fertilizer in many aquatic and terrestrial ecosystems.
However, under conditions of warm water and high nutrient loads, particularly phosphorus and nitrogen, many species can proliferate rapidly. This leads to what are known as harmful algal blooms (HABs).
These blooms, often appearing as thick green or blue-green scums, negatively impact aquatic life by blocking sunlight and causing oxygen depletion upon decomposition. Some cyanobacteria produce potent poisons called cyanotoxins, such as microcystins. These toxins can cause liver damage (hepatotoxicity) or neurological issues (neurotoxicity) in humans, pets, and livestock that ingest contaminated water. The monitoring and management of cyanobacterial blooms is a major public health concern globally.